Gas Liquid Separation & Gas Decomposition Process

Ammonia Removal System

The function of Ammonia Removal System is to process waste water containing ammonia (NH3) and oother contaminants prior to discharge the environment. The ammonia sourced is from the chemical injection in Boiler Turbine to maintain the pH around 8-9. Ammonia content may not dispose of into environment because the concentration exceed is the limits (90 mg/m3). So that is need to remove the ammonia content before discharging to the environment. The ammonia removal process have combination in two main process, that are removal from the water and removal from the gas (feed capacity 8.8 m3/hr). Ammonia Removal System consist of two main process, that are Gas Liquid Separation & Gas Decomposition Process.

Let's start discuss about gas liquid separation in Ammonia Removal System.

figure 1. gas liquid separation block diagram

Sodium Hydroxide 30% is dosing to pH Adjuster  for achieving pre-set pH ( 12 ). By pH 12, the stripper efficiency will increase due to both ammonium ions and dissolved gas exist together as the following graphic: 

figure 2. ammonia pH equilibrium

According the above graphic:

In a waste stream, ammonia ions exist in equilibrium with ammonia.

• pH < 7, virtually all ammonia will be soluble ammonia ions
• pH > 12, virtually all ammonia will be present as a dissolved gas
• 7< pH <12, both ammonium ions and dissolved gas exist together

Before going to the stepper, feed waste water is through the Ammonia waste water heater. The function is to increase temperature until 45 C so the solubility of gas in the waste water will decrease. Low solubility means easy to be separated. The graphic below can illustrate: 

figure 3. ammonia solubility at various temperature

The next step is going to Stripper which the main process in gas liquid separation. After the pH ( 12 ) and temperature ( 45 C), the waste water enters the above of the tower (Stripper). Air from Fan enters the bottom of the Stripper and travel upward through the packing. Since the ammonia is partially present as a dissolved gas, some of the ammonia transfer from the water to the air. Below is a simple image to easily know: 

figure 4. stripper

The function packing media is place that happen separation between waste water and ammonia. Mist Eliminator is to prevent moisture out from stripper.

figure 5. liquid gas separation inside packing media

Let's start discuss about gas decomposition process in Ammonia Removal System.

The simplified block diagram of gas decomposition process is in the following:

figure 6. gas decomposition process block diagram

The decomposition of ammonia is happen in Catalytic Oxidation Reactor. In the present of high temperature condition ammonia (400 C) reacts with free oxygen to form Nitrogen, Nitrogen Oxide and Water (H2O). The reaction is helped by a catalyst to accelerate the reaction (decrease activation energy). The hot gas out from Catalytic Oxidation prior pass through the stack are cool down by utilized the heat to increase temperature of residual gas (311 C), output from Stripper. Ammonia and NO2 analyzer are provided for monitoring purpose. The reaction is following :

Polymer Preparation System

Polymer Preparation System

The kind of Polymer that usually used in water and waste water treatment is Anionic Polymer. Anionic polymerization is a type of chain growth polymerization in which an anionic initiator transfers a charge to a vinyl monomer which then becomes reactive. Each reactive monomer goes on to react with other monomers to form a linear polymer.

Let RMe be an organometalic compound such as butyl lithium (LiC4H9) that initiates polymerization and M a monomer, then the individual steps can be represented as follows:

RMe ⇔ Me+ + R-             R=chemical group"gugus".     M=Monomer, from feed water also.

R- + M → RM-

RM- + M → RM2-

RMn-1- + M → RMn-

The polymerization only stops when all monomers are consumed. For this reason, this type of polymerization is often called anionic "living" polymerization.

Anionic polymerization include -CN, -COOR, -C6H5, and -CH=CH2 → group"gugus".

Now discuss about polymer preparation system.

Polymer chemical dosing or preparation is controlled automatically. This control box is connected with liquid meter  (level transmitter) on the solution tank. Once the liquid meter reaching "the low level" it will touch and open the inlet solenoid valve. At the same time, start up the unit and start to prepare the chemical (polymer). The volume of dosage and water volume can be selected correctly. This will obtain the accurate concentration. For water or waste water treatment, the polymer solution is around 0.1% until  0.2 %. On the other hand, the liquid meter reach the "high level and its cycle process will stop while the agitator is still working.

figure 1. polymer preparation system

The main component of polymer solution system:

1. Vacuum feeding machine to transfer dry polymer powder from polymer bag to screw          feeder. It has air purifier, the whole operating process should be dust-free.                          2. Chemical dosing unit consisit of screw feeder (conveyor) which to transport the dry polymer powder out of the hopper totally. At the conveyor outlet, there is electric heater unit to prevent the moisture invading into the powder. The hopper volume is 60 liters.                  3. Dry powder pre-soaking device adopt rotating water and injection water, which can mix and soak the dry powder totally, then enter into the premixing tank.                                      4. Tank: three-tank integration, including premixing tank, homogenous matureing tank and solution storage tank. There are baffle plate and overflow weir plate among different tanks. During the chemical preparation, the chemical solution should flow from baffle plate bottom into the next tank via V type overflow weir plate. There is concealad overflow pipe in the pre-mixing tank, which is used for preventing the liquid overflowing when the liquid level acess the set point (breakdown).                                                                                                  

5. Liquid level controller, the location is in the solution storage tank to control high/low/ultra-low level point.                                                                                                    6.  Agitator, there are agitators on the top of pre-mixing tank and mature tank. It can not only promise uniform solution, but also not damage to the polymer molecule.                        7. On-line dilution system, it's the source of water for dilute the dry polymer powder. It's consist of filter,pressure gauge,elbow,solenoid valve, manual stop valve,flow meter.        

8. Electric control cabinet: PLC cabinet is equipped with auto electrical elements, including manual/auto knob.

Electro-Chlorination System

Electro-Chlorination System

Electro - chlorination plant aims to prevent the deposition of marine organism such as barnacle, sea mussel and other shellfishes in the cooling water system, by directly electrolyzing seawater, without using any chemicals. The cooling water system is used to cool the some equipment in main or other process.

The following is the simplified Electro-Chlorination System:

Figure 1. electro-chlorination system

The basic reaction that happen in Electro- Chlorination System is electrolysis. The electrolysis is ia technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential.

figure 2. electrolysis process

Where as the reaction that happen in Electro-Chlorination System are the following:

1. Anode (+ pole) is happen oxidation which negative ion (Anion or Cl-) will be oxydized (Anion will go to Anoda) 
2. Cathode (-pole) is happen reduction which positive ion (cation) will be reduction (cation will go to cathode). Due to the Na+ is strong metal as 1 A group in Periodical Table of the Elements, H2O or water will be reduced in Cathode.

The reaction is below:

The injection of chlorine ( NaOCl ) is in two place, one is in intake pit, second is in sea water. Keep the chlorine product concentration and flow are enough so the free residual chlorine is 1 ppm ( during continues dosing) and 3 ppm ( during shock dosing). The chlorine injection point is in the following place:

figure 3. chlorine injection point

 To calculate the Chlorine Production Efficiency, can be used the following formula:

If the efficiency is low (less than 75%), it's mean the Calcium and Magnesium deposits accumulated on the cathode of electrolyzer. It shall result in decreased efficiency and increase power consumption so need periodically acid cleaning (al least once a month base on actual fouling degree).Use HCl 10% as chemical cleaning agent because can dissolve Mg(OH)2 and CaCO3.

Sludge Processing and Disposal

Sludge Processing and Disposal

It should be appreciated that the sludges consist mainly of water and that dewatering is the first and most important requirement in sludge processing.  The cost of treating the sludges, particularly for wastewaters, is a major component of the total cost of treatment, and the effects of the final disposal methods and return flows from sludge treatment can have significant implications for the preceding processes.

The main objective of wastewater treatment is to reduce the pollution load on receiving waters.  The treatment processes concentrate some of the impurities in a sludge along with the microbial excess biomass.  Water treatment also produces a sludge from the chemical coagulation and separation of impurities.  The treatment and disposal of these sludges should be considered as an integral part of the treatment process.  

Objectives

The main objectives of sludge treatment are:

1. Reduction in the volume of sludge for disposal by removing some of the water

2. Collection of by-products which may be used or sold to off-set some of the costs of sludge                   treatment

3. Disposal of the sludge in a safe and aesthetically acceptable manner

4. Destruction of pathogenic organisms

5. Stabilization of the organic matter contained in the sludge.

Methods

Sludge treatment and disposal at any particular location may comprise any or all of the steps as outlined below.

1. Concentration - reduction in the volume of sludge to be treated by encouraging the sludge to compact to a higher solids content

2. Treatment - to stabilise organic matter, destroy pathogens and/or yield by-products

3. Dewatering and drying - removal of water, thus reducing sludge volume. Sludges with less than        80 per cent moisture content are usually spadeable

4. Disposal - the only places where sludge can be disposed of are into the air, onto land or water.            Whether or not the impact on the receiving environment is legally, aesthetically and ecologically        acceptable depend on both the degree of treatment provided and the method of dispersing the              sludge into the environment.

Table 1. Treatment and disposal options for wastewater sludges

Thickening	Stabilization	Dewatering	Partial disposal	Ultimate disposal
Gravity	Anaerobic digestion	Drying beds	Incineration	Sanitary landfill
Flotation	Aerobic digestion	Filter press	Pyrolysis	Crop land
Centrifuge	Lagooning	Centrifuge	Wet air  oxidation	Ocean
Elutriation	Heat treatment	Vacuum filter Belt press Lagooning	Composting

Here is only describing the Thickening process by Gravity, Flotation dan Centrifuge.

GRAVITY THICKENING

A typical waste sludge thickener is illustrated in Figure 1.  The tank resembles a circular clarifier except that the depth/diameter ratio is greater and the hoppered bottom has a steeper slope.  A bridge fastened to the tank walls supports a truss-type scraper arm mounted on a central shaft.  Sludge enters at the centre behind a circular baffle that directs it downward, and supernatant overflows a peripheral weir.  Settled solids are gently agitated by slow rotation of the scraper to dislodge gas bubbles, prevent bridging of the solids, and move slurry toward a central well for withdrawal.  Feed is provided continuously while the underflow may be extracted intermittently for further processing.

Figure 1. Gravity Thickener

FLOTATION THICKENING

Air flotation is most applicable in concentration waste-activated sludges and pretreatment of industrial wastes to separate grease or fine particulate matter.  Fine bubbles to buoy up particles may be generated by air dispersed through a porous medium, by air drawn from the liquid under vacuum, gases released  by electrolysis, or by air forced into solution under elevated pressure followed by pressure release.  The latter, called dissolved-air flotation, is the process employed most frequently in thickening sludges because of its reliable performance.

The major components of a typical flotation system are sludge pumps, chemical feed equipment to apply polymers, an air compressor, a control panel, and a flotation unit.  Figure 2 is a schematic diagram of a dissolved-air system.  Influent enters near the tank bottom and exits from the base at the opposite end.  Float is continuously swept from the liquid surface and discharged over the end wall of the tank.  Effluent is recycled at a rate of 30-150% of the influent flow through an air dissolution tank to the feed inlet.  In this manner, compressed air at 700-1000 kPa is dissolved in the return flow.  After pressure release, minute bubbles with a diameter about 80mm form and attach to solid particles and become enmeshed in sludge flocs, floating them to the surface.  The sludge blanket, varying from 200-600mm thick, is skimmed from the surface.  Flotation aids are introduced in a mixing chamber at the tank inlet.

Figure 2 Dissolved Air System

CENTRIFUGATION

Centrifuges are employed for both dewatering sludges and thickening waste slurries for further processing.  Applications include sludges difficult to dewater by gravity separation, such as alum coagulation residues and waste-activated sludge and lime-softening precipitates prior to recalcining.

All centrifuges have the same basic operating principle.  Solids are removed from the waste stream flowing through the machine under the influence of a centrifugal field of 100-600 times the force of gravity.  Particles are deposited against the spinning solid bowl while the overflow is a clear liquid supernatant.  The fundamental difference in centrifuges is the manner of solids collection and discharge - the method of discharge determines the size and nature of the particles removed by a particular unit.  Material encountered in wastes includes a broad range of granular, fibrous, flocculent, and gelatinous solids that differ in settling and compaction characteristics.  Therefore, the type of centrifuge adopted is determined by the particular waste as well as discharge requirements of supernatant clarity and cake dryness.  The two most popular types for handling sanitary wastes are the scroll or conveyor centrifuge and the imperforate basket.

Scroll centrifuges can handle large quantities of fairly coarse solids (Fig. 10).  Two principal elements are a rotating solid bowl in the shape of a cylinder with a cone section on one end and an interior rotating-screw conveyor.  Feed slurry enters at the centre and is spun against the bowl wall.Settled solids are moved by the conveyor to one end of the bowl and out of the liquid for drainage before discharge while clarified effluent discharges at the other end over a dam plate.  This system is best suited for separating solids that compact to a firm cake and can be conveyed easily out of the water pool.  

Figure 3. Centrifuge

Corrosion Treatment by AVT (All Volatile Treatment) and CWT (Combined Water Treatment)

Corrosion Treatment by AVT (All Volatile Treatment) and CWT (Combined Water Treatment)

The purpose of water quality control (AVT and CWT) is getting proper water to minimize deposit and reduce corrosion in thermal power plant. Hence safe and reliable operation will increase and unit availability will be increase also.

Corrosion

Corrosion in water tube is influenced by the following condition:

1. Improper Dissolved Oxygen Content
2. Dissolved Mineral (impurity)
3. Improper pH (acid, pH < 7 , tend to corrosion)
4. Temperature
   

Figure 1. Basic corrosion reaction

Corrosion Reaction :

Fe + O2 + ½ H2O  →  Fe(OH)2

1. Corrosion by Oxygen

The most common form of corrosion is caused by the presence of oxygen. The most common method of removing oxygen is by the dearating feed water heater or condenser. For further  assurance to remove oxygen is by adding chemical oxygen scavenger ( Hydrazine /N2H4 ) after condensate polishing system.

N2H4 + O2   →     N2 + 2H2O   

      

2. Corrosio by pH

Ammonia or Amines used for pH control to solute mineral

3. Corrosion by Impurities

An oxygen differential cell is rarely the sole cause of metal loss under deposit. Coexisting with most oxygen differential cell are the corresponding Chloride and Sulfate concentration cell. These coexist because Chloride and Sulfate ions penetrate the deposit or crevice and concentrate. The deposit behave like a semi-permeable membrane, as the iron ions (Fe2+) leave the anodic surface, the Cl- + SO42- anions diffuse through the deposit to maintain neutrality, resulting in a build up of electrolyte under deposit. This further accelerates the corrosion already taking place because of the oxygen differential cell.

4. Corrosion by Temperature

As a general rule, each 15 0F(8 0C) increase in temperature doubles the rate of chemical reaction. Therefore, temperature increase the speed of corrosion.

              

Figure 2. Corrosion by Temperature

Magnetite ( Fe3O4 ) is normally formed in the inside of pipe.

Fe + 4H2O      →    Fe3O4 + 4H2           

Whereas Fe(OH)2 can also converted to Magnetite at the temperature above 100 0C

3Fe(OH)2     →       Fe3O4 + H2  + 2H2O               

Corrosion Treatment by Chemical Injection (AVT-CWT)

AVT (All Volatile Treatment): Conditioning of feed water and boiler water by adding volatile alkalizing chemical such as Ammonia, Oxygen scavenger(Hydrazine) etc.

     LO (Low Oxidizing) → < 5 µg/L

     Oxygen scavenger is not used, Oxygen is removed only by Condenser and Deaerator. 

CWT (Combined Water Treatment): Conditioning of feed water and boiler water by adding Ammonia & Oxygen.

Figure 3. Potential pH Diagram of Fe

qThe Chemical injection system is to inject chemicals to the plant cycle water to maintain appropriate pH value and to create “protection film” on the inside surface of the boiler heat transfer tubes to avoid corrosion.

qCWT (Combined Water Treatment) or Oxygenated Treatment (OT) is provided for the cycle water during the plant normal operation where ammonia and oxygen are injected in the cycle water. Ammonia is injected for the purpose of pH control (around 8.5 to 9.3) and is injected at the outlet of condensate polishing system. Oxygen is injected for the formation of “Hematite (Fe2O3)” which is more stable protective film, on the surface of boiler tube.

qThe AVT (LO) (All Volatile Treatment - low oxidizing) is considered during the start up, pH is controlled to around 9.3 to 9.7 by injecting ammonia to form a protective film of magnetite (Fe3O4).

qUnder AVT (LO) operation, pH is controlled to around 9.3 to 9.7 by injecting ammonia to form a protective film of magnetite (Fe3O4) under condition of degasified (deaerated) cycle water. Ammonia is injected at the outlet of condensate polishing system.

                                                                        Figure 4. Inside tube during AVT-CWT

The Chemical Injection location is the following:

The Case Study:

Question:

In the case that the conductivity (cation pass) at the boiler feed water exceeds > 0.2 μS/cm, oxygen injection is stopped under the oxygen treatment. What kind of problems will occur, if oxygen injection into boiler feed water continues in the case of the conductivity (cation pass) is higher than > 0.2 μS/cm?

Answer:

The case in which conductivity (cation pass) at the boiler feed water is higher than > 0.2 μS/cm indicates the increase of contaminants, it means the amount of anions such as chloride and sulfate ion increases. In this case, the iron oxide protective film is destroyed and corrosion will progresses. The process must back to AVT.